drivers/video/backlight/l4f00242t03.c: check return value of regulator_enable()

regulator_enable() is marked as as __must_check.  Therefore the return
value of regulator_enable() should be checked.  Also, this patch checks
return value of regulator_set_voltage().

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>